There has long existed a need for rapidly determining the effect of a chemical agent on the growth of bacteria, especially medically significant bacteria, and especially disease causing bacteria. For example, there has long existed a need to rapidly determine the susceptibility of pathogenic bacteria to antibiotics.
It is well known that disease-causing bacteria have varying degrees of susceptibility to bacteriostatic and bactericidal agents (hereinafter referred to as "antibiotics"), and that most effective control of a bacterial infection would be achieved by administering to the patient (human or animal) the most effective antibiotic for the particular bacterial infection in question. However, because of the wide variety of bacteria and antibiotics, this has not been practicial for the practicing physician (or veterinarian) because the risk of adverse consequences through selection of the wrong antibiotic is too great. As a consequence, the physician usually selected a broad spectrum antibiotic and hoped that it would be effective against the particular bacteria in question.
Attempts have been made to refine and improve diagnostic procedures for isolating from a patient and characterizing disease-causing bacteria, and ascertaining the most effective antibiotic against that particular bacterial population. However, the procedures developed to date are quite time-consuming, requiring at least 30 hours from collection of a specimen to final report. As a consequence, the practicing physician must still resort to the use of broad-spectrum antibiotics.
The previously known techniques for rapidly determining bacterial susceptibility to antibiotics were based upon optical principles. That is, a specimen was collected, divided into aliquots and subjected to a battery of antibiotics. The resulting samples were then scanned optically, using either turbidometric or light scattering techniques, to ascertain the degree to which a particular antibiotic inhibited the growth of the bacteria, as compared with a control sample. Regardless of the specific optical method employed, such procedures required large samples to provide sufficient bacterial cells to permit optical detection and differentiation, as well as substantial incubation periods during the assay to again generate sufficient numbers of cells to enable accurate comparisons between control and test aliquots. This procedure could consume 48 hours or more. Even with the introduction of automated instruments, which materially reduced the time required to effect analysis of the samples, the procedure still normally required at least 30 hours. Accordingly, although such procedures have been employed in clinical laboratories to follow the course of a bacterial infection in a population, they have not been of practical value to the practicing physician or veterinarian in the diagnosis and treatment of bacterial infections in individual patients.
The known optical techniques are also deficient in another respect. As an aid to more effective treatment, it would be desirable to know not only the most active antibiotic, but also the minimum inhibitory concentration (or "MIC") of the antibiotic against the bacteria in question. However, optical devices cannot be reliably used to determine MIC's because most antibiotics severely affect the morphology of sensitive bacteria during the first four to six hours of exposure. In addition, concentrations of antibiotics close to or slightly below the MIC often have the greatest effect on bacteria morphology. The optical properties of the morphologically altered bacteria are different from those of the untreated control bacteria; consequently, optical techniques for determining MIC may generate inexact data.
It has been proposed in U.S. Pat. No. 3,676,679 to detect the presence of bacteria in a specimen by culturing the specimen on a culture medium including a .sup.14 C-containing carbon source fermetable to produce gaseous carbon dioxide, such as .sup.14 C-labelled glucose; collecting the gaseous atmosphere over the culture; and measuring its radioactivity. If active bacteria are present in the specimen, radioactively-labelled carbon dioxide will be present in the gaseous atmosphere. However, the patentee contemplated use of this method solely to the detection of the presence of active bacteria. It was not proposed that this procedure be employed to ascertain susceptibility to antibiotics.